During historic and prehistoric times the southern slope of Irazu Volcano (Costa Rica) has been affected by several mass movements. The stratigraphic record has revealed the presence of extensive debris flow and debris avalanche deposits. The last eruptive activity (1963-1965) was characterized by at least 40 lahars, one of which caused the death of 20 people and heavy damage to the city of Cartago. The lahar events were triggered by heavy rains with the initial involvement of the superficial ash cover. As the flows moved downstream, the detrital substratum of the Reventado River valley (made up by a 50 m thick debris avalanche deposit) was strongly affected. The aim of the first step of the research has been to provide useful data to reconstruct the triggering conditions of debris avalanche and lahars and to evaluate both their magnitude and mobility. For this purpose geological and volcanological investigations coupled with the physicalmechanical characterization of the material involved in the mass movement have been carried out. The rock samples collected from the detachment area of the avalanche are characterized by a sharp decrease of the compressive strength as a function of the degree of the hydrothermal alteration. The complete destruction of the silicate lattice acted by the hydrothermal fluids has produced amorphous silica, clay minerals and sulfates (gypsum, anhydrite). As the alteration degree increases, the compressibility modulus value decreases from those of rocks to those of loose material. As a consequence, the catastrophic failure probability of sectors of the volcanic edifices is enhanced with likely large-scale mass movements. Samples collected from deposits involved in small-scale events (lahars) were analyzed to determine density, grain size distribution and friction angle. These parameters will be used to performs a dynamic analysis of the 1963-65 events and will be processed to develop a forecasting model of these events.
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